This invention relates in general to an electrophotographic imaging system, and more specifically, to electrophotographic imaging members containing two selenium containing layers and a method of utilizing such members.
The formation and development of images on the imaging surfaces of electrophotographic imaging members by electrostatic means is well known. One of the most widely used processes being xerography described, for example, in U.S. Pat. No. 2,297,691 to Chester Carlson. Numerous different types of photoreceptors can be used in the electrophotographic imaging process. Such electrophotographic imaging members may include inorganic materials, organic materials, and mixtures thereof. Electrophotographic imaging members may comprise contiguous layers in which one of the layers performs a charge generation function and the other layer performs a charge carrier transport function or may comprise a single layer which performs both the generation and transport functions.
Electrophotographic imaging members based on amorphous selenium have been modified to improve panchromatic response, increase speed and to improve color copyability. These devices are typically based on alloys of selenium with tellurium. The selenium electrophotographic imaging members may be fabricated as single layer devices comprising a selenium-tellurium alloy layer which performs both charge generator and charge transport functions. the selenium electrophotographic imaging members may also contain multiple layers such as, for example, a selenium alloy transport layer and a contiguous selenium-tellurium alloy generator layer. These multiple layer electrophotographic imaging members containing a selenium-tellurium alloy generator layer are characterized by varying degrees of electrical instability during cycling. For example, multiple layer electrophotographic imaging members containing a selenium-tellurium alloy generator layer containing about 10 percent by weight tellurium and a selenium-arsenic alloy transport layer exhibit significant levels of residual cycle-up which may be further agravated by cycle rate, thermal cycling at elevated temperatures and by undesirable interactions with lamps and corotrons adjacent the electrophotographic imaging member. The addition of arsenic to a generator layer composition of selenium-tellurium can increase photoreceptor life about 1.5 to about 2 times that of a generator layer composition containing only selenium-tellurium. When arsenic is added to a generator layer composition of selenium-tellurium, the crystalization resistance of the electrophotographic imaging member is increased. These electrophotgraphic imaging members exhibit increased life under conditions of high humidity and/or high temperature which usually promote crystallization of a non-arsenis bearing selenium alloy layer. Such crystallization problems are particularly accute in office buildings in tropical regions where the office buildings are not air conditioned or where the air conditioning is turned off in the evening to conserve energy. However, addition of arsenic to a generator layer containing selenium-tellurium generally produces an increase in residual potentials and residual cycle-up. Residual cycle-up is the cumulative development of increasing levels of residual voltage with cycling. Residual voltage is that potential measured at the surface of the photoreceptor following photodischarge of the photoreceptor by high levels of light exposure during the erase cycle. The residual voltage is a reflection of the existence of positive charge (in the case of a positive charging system) trapped in the bulk of the photoconductive layers or at interfaces between layers in a photoconductive device. The rate of residual cycle-up and its ultimate saturation value is generally observed to increase with increasing cycle rate. Equilibration of the photoreceptor at temperatures above room temperature either during photoreceptor storage or during machine operation also generally leads to a temporary enhancement of residual cycle-up, both its rate of increase and its saturation value. Similarly, exposure of electrophotographic imaging members containing a selenium-tellurium alloy generator layer to radiation in the 600 to 700 nanometer range, e.g. light from tungsten or fluourescent room lights, during installation of the imaging member in a copier, duplicator or printer can cause a marked increase in cycle-up during subsequent use due to bulk absorbed radiation. More specifically, the presence of an arsenic concentration in both the transport and generator layers can lead to arsenic diffusive transport across the boundary between the transport layer and generator layer thereby producing extensive charge trapping. Such trapping can induce enhanced potential dark decay which in turn induces copy quality degradation evidenced by positive ghost image formation. Ghost imaging is the retention of an image from a prior copy cycle.